DESCRIPTION OF THE PRIOR ART
Hastelloy X is a nickel-chromium-iron-molybdenum alloy that possesses an exceptional combination of oxidation resistance and high temperature strength. It has wide use in gas turbine engines for combustion zone components such as transition ducts, combustor cans, spray bars and flame holders as well as in afterburners, tailpipes and cabin heaters. It is also used in industrial furnace applications because it has unusual resistance to oxidizing, reducing and neutral atmospheres.
Hastelloy X is typically available in cast or wrought forms but is also available as a powder metallurgy (PM) product. Conventional PM processing of Hastelloy X includes press and sinter, which results in compacts limited to simple geometric shapes such as cylinders that are not fully dense. Additional processing, such as hot isostatic pressing (HIP), can bring densities to near 100% of theoretical density.
Metal-injection-molding(MIM) is recognized as a premier forming method for complex, shapes. It affords significant advantages over other forming methods due to its capability of rapidly producing net shape, complex parts in high volume. Initially, MIM comprised the step of mixing metal powder with a dispersant and a thermoplastic organic binder of variable composition. The molten powder/binder mixture was heated during the injection molding process and injected into a relatively cold mold. After solidification, the part was ejected in a manner similar to injection molded plastic parts. Subsequently, the binder was removed and the part was densified by a high temperature heat treatment. There were a number of critical stages in this process, which included the initial mixing of the powder and binder, the injection of the mixture into the mold, and the removal of the organic matrix material used as the binder. One of the main disadvantages of the initial MIM process is the removal of the organic vehicle. Currently, with organic binder MIM processes, the cross section limit of a part for fine particle sizes is typically less than 1/4 inch. If the cross section of the part exceeds that limit, the binder removal process will lead to defects, pinholes, cracks, blisters, etc. Binder removal takes place by slow heat treatments that can take up to several weeks. During debinding at elevated temperatures, the binder becomes a liquid, which can result in distortion of the green part due to capillary forces. Another disadvantage of the initial MIM process is the tendency for the relatively high molecular weight organic to decompose throughout the green body, causing internal or external defects. The use of solvent extraction, wherein a portion of the organic is removed using an organic or supercritical liquid, sometimes minimizes defect formation. Solvent extraction causes difficulties because the remainder still needs to be removed at elevated temperatures, resulting in the formation of porosity throughout the part, which facilitates removal of the remaining organic material. During binder removal, part slumping can pose problems, especially for the larger particle sizes if the green density/strength is not high enough.
MIM offers certain advantages for high volume automation of net shape, complex parts. However, the limitation of part size and the excessive binder removal times, along with a negative environmental impact resulting from the debinding process have inhibited the expected growth of the use of this technique.
Some improvements, such as the use of water based binder systems, have been made to the initial MIM process. Hens et al. developed a water leachable binder system as described in U.S. Pat. No. 5,332,537. The injection molding feedstock is made with a tailored particle size distribution (to control the rheology), a PVA-based majority binder, and a coating on each of the binder particles. During molding, these coatings form necks which give the part rigidity. After injection molding, there is a water debind that lasts several hours. After the remaining binder is cross-linked by either UV or chemical methods, the part undergoes a thermal debind, which takes 8-12 hours for a part such as a golf club head. Other aqueous-based binders contain either polyethylene glycols, PVA copolymers, or COOH-containing polymers. BASF has developed a polyacetal-based system that is molded at moderately high temperatures after which the binder is removed by a heat treatment with gaseous formic or nitric acid. The acid treatment keeps the debind temperature low to exclude the formation of a liquid phase and thus distortion of the green part due to viscous flow. The gaseous catalyst does not penetrate the polymer, and the decomposition takes place only at the interface of the gas and binder, thereby preventing the formation of internal defects. These improvements are limited by the requirement for separate binder removal furnaces and times, depending on the part size. There are environmental issues as well with removal of the large amount of wax/polymer in the form of fire hazards and volatile organic compound discharge.
An injection molding process using agar as an aqueous binder has been developed by Fanelli et al, as described in U.S. Pat. No. 4,734,237. This binder system applies to both ceramic and metal powders. It also includes the use of agarose or derivatives of polysaccharide aqueous gels. The advantage over state-of-the-art wax-based binder technology is the use of water as the fluid medium versus wax. In feedstocks prepared according to this technology, water serves the role of the fluid medium in the aqueous injection molding process, comprising roughly 50 volume % of the composition, and agar provides the "setting" function for the molded part. The agar sets up a gel network with open channels in the part, allowing easy removal of the water by evaporation. By contrast the Hens et al system requires a solvent debind to attain similar open channels in the part. The agar is eventually removed thermally; however, it comprises less than 5 volume fraction of the total formation, and debind times are rapid compared to wax/polymeric debind systems. This is an advantage over the Hens et al system.
This agar based aqueous binder is especially applicable for the production of stainless steel components using MIM. Due to the easy removal of the aqueous based binder and its relatively low level of carbon, as compared to wax or polymeric binder systems, debinding and sintering schedules have been developed by Zedalis et. al (U.S. patent application Ser. No. 09/141,444) which impart little or no additional carbon to stainless steel alloys such as 316L, 410 and 17-4PH. Moreover, the agar based binder and its associated carbon are removed in a simple one step, air debind consisting of relatively short debind times of approximately 1/2 to 2 hours. In contrast, wax or polymer based binders require several step debinding processes in which each debind step often takes many more hours. Accordingly, the short air debind times of the agar-based feedstocks are economically advantageous.
Nickel based alloys have not traditionally been exploited using MIM processing. Valencia et al ("Superalloys 718, 625, 706 And Various Derivatives"; E. A. Loria; Minerals, Metals And Materials Society, 1994; page 935) have applied the wax/polymer binder systems to MIM of the nickel superalloys 625 and 718 and have reported acceptable mechanical properties. However, production of those components suffered from the limitations of the wax/polymer debind system, i.e. long debind times resulting in uneconomical processing and part size limitations.